The present invention relates to a method for removing organic sulfur from a liquid composition containing sulfur-containing organic compounds. This method can be employed in the production of transportation fuels and, in particular, gasoline and diesel fuels produced by the fluidized catalytic cracking of a feedstock.
The auto industry has long been faced with the problems associated with sulfur in fuels and, in particular, transportation fuels. These problems have largely crystallized around three areas, first, the degradation or poisoning of catalyst employed in emission control systems. Second, the problems associated with sulfur monitoring, i.e., false sensor readings relating to catalyst performance. To this end, recent amendments to the Clean Air Act will require the automotive industry to equip future vehicles with onboard diagnostic devices that can measure the effectiveness of catalyst emission controls for 100,000 miles. The diagnostic devices will measure oxygen storage capacity to determine catalyst efficiency. The sulfur in gasoline can interfere with this oxygen storage measurement causing the sensor to show an erroneous decrease in catalyst efficiency. In order to prevent these erroneous readings, the sulfur content of gasoline should be no more than 50 ppm.
Third, sulfur dioxide emissions are a precursor to the formation of sulfate aerosols which are believed to play a substantial role in the total amount of ambient "fine particulate" matter. Moreover, recent epidemiological studies cited through the EPA suggest that sulfate aerosols are at least a contributory constituent of 2.5-micron particulate matter. Thus, the need to reduce sulfur emissions is also tied -at least in part-to ongoing efforts to reduce the emission of 2.5-micron particulate matter.
In modern refineries, fuels are produced from a number of streams. A primary source of sulfur-containing organic compounds in the gasoline pool is FCCU gasoline, i.e., gasoline produced in a fluidized catalytic cracking unit. The primary source of sulfur in FCCU fuels is thiophenic sulfur comprised of dibenzothiophene, benzothiophene, and thiophene and their various derivatives.
In order to produce a low sulfur FCCU fuel, the art has focused on either (i) hydrotreating the FCCU feed or (ii) treating the FCCU product for sulfur removal. Neither of these techniques, however, have been entirely effective.
The hydroprocessing of FCCU feed has been in commercial practice for over thirty years. In this regard, FCCU fuels have a high octane number due -at least in part-to the olefin content of the lighter, lower-boiling fraction. The hydrotreatment of FCCU fuels for sulfur removal can, however, result in a significant loss of efficiency. For example in gasolines, R+M.sub.2 octane losses of 3-10 numbers or more can result depending on the severity of the hydrotreatment.
Moreover, hydrotreatment to a sulfur level of, e.g., 50 ppm, can be cost prohibitive. In particular, the costs of decreasing sulfur content from 500 ppm to 200 ppm by desulfurizing heavy FCCU fuels can be on the order of about one cent per gallon. However, the next increment of decrease in sulfur, e.g., from 200 ppm to 50 ppm, can be on the order of four cents a gallon. This cost is only further increased in those instances where processing, e.g., isomerization, is needed to offset fuel efficiency loss during hydrotreatments.
Separately, microorganisms have been employed in processes for removing organic sulfur from certain thiophenic compounds.
The biological pathways employed are either "carbon-destructive" which results in the overall degradation of the compound or "sulfur-specific" which results in the selective attack on the sulfur atom. A preferred sulfur specific process involves four enzymatic steps of "sulfoxide-sulfane-sulfonate-sulfate" and is hence termed the "4S pathway". The 4S pathway is illustrated in FIG. 1a-1d. Attention in this regard is directed towards the article entitled "Desulfurization of Coal: The Microbial Solution", Trends Biotechnol., 7,97-101, April 1989, by J.J. Kilbane, that discusses the 4S pathway in detail. Attention is also directed towards U.S. Pat. Nos. 5,496,720; 5,510,265; and 5,529,930, which are all incorporated by reference in their entirety for all purposes.
For example, non-commercial techniques for the desulfurization of diesel fuel have employed biocatalysts such as Rhodococcus erythropilis, which catalysts produce enzymes capable of selectively oxidizing sulfur found in heterocyclic sulfur-containing organic compounds, such as dibenzothiophene (DBT), to sulfone. The sulfur is then "clipped" from the molecules to produce an oxygenated, sulfur-free derivative of biphenyl and inorganic sulfate.
Although biocatalytic oxidation by way of the 4S pathway is capable of removing sulfur in diesel fuels, it has been severely limited both in terms of its applicability and its effectiveness. In particular, biodesulfurization as it exists in the art is currently focused on the desulfurization of diesel fuels not gasolines. Moreover, biodesulfurization has suffered from the same cost and octane problems that can be associated with traditional hydroprocessing. In light of these problems, it has not found commercial success.
Accordingly, the need still exists for a method of removing sulfur-containing compounds from feed streams for fuels and, in particular, FCCU fuels.